1. Field of the Invention
This invention relates to apparatus providing protection from faults, including arc faults and ground faults, in two line electric power distribution systems, and particularly to apparatus that trips circuit breaker poles in both lines in response to an arc fault or ground fault in either line.
2. Background Information
Branch circuits in electric power distribution systems often share a common neutral conductor to reduce the wiring required. Such multi-wire branch circuits are often referred to as “home runs”. Home runs are only permitted under certain conditions. Generally, they are permitted as long as the two line conductors are energized by separate phases or by a center tapped single phase to avoid overloading the neutral conductor, and as long as all ungrounded conductors are disconnected simultaneously at the panel board where the branch circuits originate. This simultaneous disconnection of the ungrounded conductors can be accomplished with a two pole disconnect, two single pole circuit breakers with a handle tie, or a two pole circuit breaker.
Typically, such multi-wire branch circuits are provided with short circuit and overload protection by the tied single pole breakers or the two pole breaker. Only the two pole breaker can also provide ground fault protection by the addition of a common ground fault detector. Recently, there has been an increased interest in providing protection from arc faults. Arc faults are intermittent high impedance faults which can be caused for instance, by worn insulation, loose connections, broken conductors, and the like. Because of their intermittent and high impedance nature, they do not generate currents of sufficient instantaneous magnitude or sufficient average current to trigger the thermal-magnetic trip device that provides the short circuit and overload protection.
U.S. Pat. No. 5,896,262 discloses apparatus for detecting faults in multi-wire branch circuits where both poles are tripped simultaneously in response to all faults, overcurrent, arcing and ground faults, regardless of which line conductor is involved. The thermal-magnetic devices of the two poles are mechanically interconnected so that when one pole trips the other is also tripped. A common trip coil is energized by either of separate arc fault detectors associated with the two circuit breaker poles or by a common ground fault circuit. As each of the arc fault circuits is referenced to an associated line conductor and the single ground fault circuit is referenced to neutral, the arc fault trip signals are isolated by opto-isolators from the electronic switch that turns on the coil.
The arrangement of U.S. Pat. No. 5,889,643 complies with the general conditions for a multi-wire branch circuit discussed above where all ungrounded conductors must be opened simultaneously. This is because the two line conductors can be connected to a single appliance, such as an electric stove, for instance, to provide 220 volts, or to two separate outlets in a common receptacle. However, where the two line conductors with the common neutral, are connected to independent loads, both circuits do not have to be interrupted simultaneously in response to an overcurrent condition (a short circuit or overload) in one line conductor. By only interrupting current in the affected line conductor disruption of service is minimized. On the other hand, an arcing fault occurs because of a wiring failure. This wiring may be in the multi-wire cable, and therefore, both poles should be tripped. U.S. Pat. No. 6,014,297 is directed to apparatus which is adapted to this situation. The thermal-magnetic trip devices are not mechanically coupled so that an overload or short circuit will only open the affected line conductor. Both circuit breaker poles are opened in response to an arc fault or ground fault by separate coils that actuate the breaker pole associated with each of the line conductors. Thus, detection of an arc fault by either arc fault circuit or the single ground fault circuit results in energization of both trip coils. However, again, as the arc fault circuits are separately referenced to the two line conductors and the ground fault circuit is referenced to the neutral conductor, opto-isolators are needed to electrically isolate the trip signals.
While the apparatus of U.S. Pat. Nos. 5,889,643 and 6,014,297 perform their intended functions, there is room for improvement in protection in multi-wire branch circuits.